Happy99 was a worm that appeared in early 1999. It used to attach itself to emails and display fireworks.

The Melissa worm, released in 1999, was meant for Microsoft Word and created a lot of network traffic.

The iloveyou worm, also known as the love bug worm, was released in 2000. It was written in VBScript by a Filipino student. It was known to spread to millions of computers in a short amount of time.

The Code Red worm was known to spread in Microsoft systems in 2001. The Nimda worm was the next famous one, in the same year, and seemed to infect Microsoft operating systems.

 SQL Slammer was seen in 2003 and spread through the entire internet using by exploiting a bug in Microsoft SQL Server.

Baggle and Brontok were mass-mailing worms seen in the 2000s.

Conficker was another notorious piece of malware seen in 2008 which was known to infect Microsoft servers.